Attenuating type phase shifting mask and method of manufacturing thereof

ABSTRACT

An attenuating type phase shifting mask according to the present invention includes an attenuating type phase shifting pattern, and an attenuating type auxiliary phase shifting pattern having a transmitting portion and a phase shifter portion formed at a predetermined position at the periphery of attenuating type phase shifting pattern, wherein attenuating type auxiliary phase shifting pattern includes a pattern having a resolution smaller than a limit of resolution of an exposure apparatus. Whereby, exposure of the region around a normal exposure region is prevented, and also exposure of the region adjacent to the exposure region is prevented when conducting exposure successively with a substrate moved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an attenuating type phase shiftingmask, and more particularly, to a pattern of attenuating type phaseshifting mask and a method of manufacturing thereof.

2. Description of the Background Art

Recently, high integration and miniaturization of the semiconductorintegrated circuit has been developed rapidly. Accordingly, fineprocessing of the circuit pattern formed on the semiconductor substratehas been developed rapidly. Particularly, photolithography has beenrecognized broadly as basic technology in formation of the pattern.Although various developments and improvements have been carried out sofar for photolithography, miniaturization of the pattern is still ongoing and the need for improvement of the resolution of the pattern hasbeen more and more increased.

Generally, the limit of resolution R (nm) in photolithography utilizingthe demagnification exposure method can be represented as follows:

    R=k.sub.1.λ/(NA)

where λ is a wavelength (nm) of light used, NA is the numerical apertureof a lens and k₁ is a constant depending on resist process.

As can be seen from the above expression, values of k₁ and λ should bedecreased and the value of NA should increased in order to improve thelimit of resolution. In other words, the constant depending on resistprocess should be decreased, while the wavelength should be shortenedand NA should be increased. However, it is difficult technically toimprove the light source and lenses, and by shortening the wavelengthand increasing NA, the depth of focus δ (δ=k₂.λ/(NA)²) of light becomesshallower causing decrease of the resolution.

Referring to FIGS. 16(A), 16(B), and 16(C), a cross section of the mask,an electric field of exposure light on the mask and intensity ofexposure light on a wafer when a conventional photomask is utilized willbe described.

Referring to FIG. 16(A), a cross sectional structure of the photomaskwill be described. A metal mask pattern 20 made of chromium or the likeis formed on a quartz glass substrate 10. Referring to FIG. 16(B), theelectric field of exposure light on the photomask matches the maskpattern. As to the intensity of exposure light on the wafer, however, itis noted as shown in FIG. 16(C) that especially when the fine pattern isutilized, beams of the light transmitted through the mask intensity witheach other at adjacent pattern images where those beams of light overlapwith each other due to diffraction and interference of light.Consequently, the difference of the light intensity on the wafer isreduced resulting in decreased resolution.

In order to solve the above-mentioned problem, the phase shiftingexposure method utilizing a phase shifting mask has been proposed in,for example, Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.

Referring to FIGS. 17(A), 17(B) and 17(C), the phase shifting exposuremethod utilizing the phase shifting mask disclosed in Japanese PatentLaying-Open No. 58-173744 will be described.

FIG. 17(A) shows a cross section of the phase shifting mask. FIG. 17(B)shows the electric field of exposure light on the photomask. FIG. 17(C)shows the light intensity of exposure light on the wafer.

Referring to FIG. 17(A), the phase shifting mask includes a chromiummask pattern 20 formed on a glass substrate, and at every other apertureof mask pattern 20, a phase shifter 60 formed by a transparentinsulating film such as a silicon oxide film is provided.

Referring to FIG. 17(B), as to the electric field of exposure light onthe photomask provided by the light transmitted through the phaseshifting mask, phases of the exposure light are alternately reversed by180°. In other words, beams of light are canceled with each other atadjacent pattern images where beams of light overlap with each other dueto interference. Accordingly, as shown in FIG. 17(C), the resolution ofpattern images can be improved, because of the sufficient difference ofthe intensity of exposure light on the wafer.

Although the above-mentioned phase shifting mask is very effective for aperiodic pattern such as lines and spaces, the mask cannot be setappropriately in case of a complicated pattern because arrangement orthe like of phase shifters is very difficult.

In order to solve the above-mentioned problem, an attenuating type phaseshifting mask is disclosed in, for example, "JJAP Series 5 Proceedingsof 1991 International Micro Process Conference, pp. 3-9" and in JapanesePatent Laying-Open No. 4-136854. The attenuating type phase shiftingmask disclosed in the Japanese Patent Laying-Open No. 4-136854 will bedescribed below.

FIG. 18(A) shows a cross section of the above-mentioned attenuating typephase shifting mask 500. FIG. 18(B) shows the electric field of exposurelight on the mask. FIG. 18(C) shows the intensity of exposure light onthe wafer.

Referring to FIG. 18(A), phase shifting mask 500 includes a phaseshifting pattern 300 which is a predetermined exposure pattern,including a quartz substrate 10 through which the exposure light istransmitted, a transmitting portion 100 formed on a main surface ofquartz substrate 10 for exposing the main surface of quartz substrate10, and a phase shifter 200 for shifting the phase of the exposure lighttransmitting therethrough by 180° relative to the phase of the exposurelight transmitting through said transmitting portion 100.

Phase shifter 200 is an absorption type shifter film of a double-layeredstructure including a chromium layer 20 having 5-20% transmittance ofexposure light, and a shifter layer 30 for converting the phase of theexposure light transmitting through transmitting portion 100 by 180°.

The transmittance of exposure light of phase shifter 200 is set to5-25%, which is appropriate for lithography, since the thickness of theresist film after development is adjusted according to the transmittanceas shown in FIG. 19.

The electric field on the mask of the exposure light transmittingthrough the phase shifting mask having the above-mentioned structure isas shown in FIG. 18(B). As to the intensity of the exposure light on thewafer, the light intensity at the edge of the exposure pattern isinevitably 0 as shown in FIG. 18(C), since the phase of the exposurelight is reversed at the edge of the exposure pattern. Consequently,there is provided sufficient difference between the light intensitycorresponding to the transmitting portion 100 and that corresponding tothe phase shifter 200, so that the resolution can be improved.

The above-mentioned conventional technology, however, has the followingdisadvantages.

FIG. 20(A) shows positions of the attenuating type phase shifting maskplaced in the exposure apparatus and a blind 70 for determining theexposure region of the exposure apparatus. FIG. 20(B) shows the electricfield of the exposure light directly under the attenuating type phaseshifting mask and blind 70. FIG. 20(C) shows the light intensity on theexposed material of the light transmitted through the attenuating typephase shifting mask and blind 70. FIG. 20(D) shows the region exposed bythe light transmitted through the attenuating type phase shifting maskand blind 70.

Referring to FIG. 20(A), the region other than a chip pattern formingregion (Lc) of the attenuating type shifting mask is covered withabsorption type shifter film 20 in which pattern processing is notconducted. In a demagnification projection, exposure apparatus blind 70which interrupts the light for determining the exposure region isprovided at a prescribed position under the attenuating type phaseshifting mask.

The aperture of blind 70 may have any width so long as it allowsexposure of the chip pattern region, and therefore the width may be thesame as the chip pattern region (Lc). However, since the position ofblind 70 is controlled by the distance of about 1000 μm (about 1 mm) andblind 70 is not positioned on the same plane of focus as the attenuatingtype phase shifting mask, the edge portion of blind 70 is not wellfocused out of focus. Thus, as shown in FIG. 20(A), the width ofaperture of blind 70 (Lb) is set to about 1000 μm wider than the chippattern region (Lc) so that blind 70 does not overlap the chip patternregion (Lc).

Accordingly, in an ordinary photomask utilizing an interrupting film ofchromium, for example, on the chip pattern, only 1/1000 of the light canbe transmitted through chromium at most, so that the light passingthrough the gap between the chip pattern region and blind 70 will notexpose the resist film on the semiconductor wafer.

In case of the attenuating type phase shifting mask, however, 5-20% ofthe exposure light passes through the gap between the chip patternregion and blind 70 as indicated by portion A of FIG. 20(B) because thetransmittance of the absorption type shifter film serving as a materialof the chip pattern is about 5-20%. Consequently, referring to FIG.20(C), a region having the light intensity of I' which is 5-20% of thetransmitted light I₀ results between chip pattern region Lc and blind 70as can be seen from the distribution of intensity of the lighttransmitted through the attenuating type phase shifting mask and throughblind 70. Thus, referring to FIG. 20(D), there is formed a region 50having the light intensity (I') and having the width of Ld of 5-20%around chip pattern region 30 (Lc×Lc).

When patterns of the attenuating type phase shifting mask aredemagnified and transferred onto the semiconductor wafer utilizing thedemagnification projection exposure apparatus having the above-describedstructure, exposure proceeds successively at a pitch of Lc which is thesize of the chip pattern FIG. 21 shows the exposed regions on thesemiconductor wafer when the wafer is exposed using the attenuating typephase shifting mask having the chip pattern of the size (Lc×Lc),utilizing the demagnification projection exposure apparatus.

In this case, since exposure proceeds at a pitch of Lc longitudinallyand transversely, there is a region 50 having the light intensity (I')of 5-20% (I') as described above around the chip pattern provided by oneexposure shot. This region 50 overlaps an adjacent region provided byanother exposure shot. As exposure is repeated successively, region 50is exposed overlapped with adjacent three regions 50 at each corner ofthe exposure region. As a result, the exposed region comes to includeregion 31 each of which is exposed with the light having the appropriateintensity I₀ plus the intensity I', which is 5-20% of I₀, and region 32each of which is exposed with the light having the intensity I₀ plusthree times the intensity I'.

In regions 31 and 32 which are exposed in such overlapped manner, when apositive resist film, for example, is exposed, the thickness of theresist film decreases after development. On the other hand, when theabsorption type shifter film having a high transmittance is used, theresist film is completely exposed so that the resist film is lost bydevelopment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an attenuating typephase shifting mask having such patterns that prevent exposure of theregion around a normal exposure region, and more specifically preventexposure of regions adjacent to the exposure region when exposure iscarried out successively with the mask moved.

In order to achieve the above object, the attenuating type phaseshifting mask according to one aspect of the present invention includesan attenuating type phase shifting pattern formed at a predeterminedposition on a photomask substrate; and an attenuating type auxiliaryphase shifting pattern having a transmitting portion and a phase shifterformed on a predetermined position at the periphery of said attenuatingtype phase shifting pattern. Also, the attenuating type auxiliary phaseshifting pattern includes patterns which are smaller than the limit ofresolution of the exposure apparatus.

According to the attenuating type phase shifting mask, images cannot beformed on the semiconductor wafer by the light transmitted through theattenuating type auxiliary phase shifting pattern because the pattern issmaller than the limit of resolution of the exposure apparatus. Further,since beams of the light transmitted through the transmitting portionand beams of the light transmitted through the phase shifter overlapwith each other and are in reverse phases, those beams cancel with eachother due to interference, so that the light intensity on thesemiconductor wafer can be reduced.

According to the present invention, values of the planar area (S₀) ofthe transmitting portion, the planar area (S_(H)) of the phase shifter,and transmittance T of the phase shifter are set such that thesubstantial light intensity on the exposed material resulting from lightintensity on the exposed material of the light transmitting through thetransmitting portion and the light intensity on the exposed material ofthe light transmitting through the phase shifter overlapped and canceledwith each other is not more than 3% of the light intensity beforetransmitting through said transmitting portion and said phase shifter.

Accordingly, the light intensity on the semiconductor wafer can becontrolled by adjusting the intensity of light transmitting through thephase shifter and the intensity of light transmitting through thetransmitting portion.

More preferably, according to the present invention, in the attenuatingtype auxiliary phase shifting pattern, the ratio of the planar area (S₀)of the transmitting portion to the planar area (S_(H)) of the phaseshifter, which is represented by S₀ /S_(H), is set approximately equalto √T of the transmittance (T) of the phase shifter.

According to the attenuating type phase shifting mask of the presentinvention, since the ratio S_(H) /S₀ is set as described above, thelight intensity on the semiconductor wafer can be set not more than 3%of the light intensity before transmitting through the transmittingportion and the phase shifter.

More preferably, according to the present invention, the attenuatingtype auxiliary phase shifting pattern is provided at the entireperiphery of the attenuating type phase shifting pattern.

Accordingly, even when a plurality of portions on the semiconductorwafer are to be exposed utilizing the attenuating type phase shiftingmask, exposure can be carried out without exposing other regions.

More preferably, according to the present invention, the attenuatingtype phase shifting pattern is rectangular and the attenuating typeauxiliary phase shifting pattern is provided in the vicinity of each offour corners of the attenuating type phase shifting pattern.

Therefore, when the semiconductor wafer is to be exposed successively inorder utilizing the attenuating type phase shifting pattern,satisfactory exposure can be carried out because no region is exposedmore than once by the light transmitting through the periphery of theattenuating type phase shifting pattern.

More preferably, according to the present invention, the planar shape ofthe transmitting portion of the attenuating type auxiliary phaseshifting pattern is rectangular.

Since the phase shifter is provided integrally on the photomasksubstrate and the transmitting portion is in the form of aperture, thephase shifting photomask can be adhered on the substrate so that theattenuating type phase shifting mask having a highly reliable structurecan be provided.

More preferably, according to the present invention, the transmittingportions and the phase shifters of the attenuating type auxiliary phaseshifting pattern are linear and arranged alternately.

Thus, the pattern can be lines and spaces so as to reduce the cost ofthe attenuating type phase shifting pattern.

In order to achieve the above object, a method of manufacturing anattenuating type phase shifting mask includes the following steps.First, an attenuating type phase shifter film having a lighttransmittance of 5-20% is formed on a transparent substrate for shiftinga phase of transmitted light by 180°.

After that, a resist film including an attenuating type phase shiftingpattern region and an attenuating type auxiliary phase shifting patternregion formed at a predetermined position at the periphery of theattenuating type phase shifting pattern region is formed on theattenuating type phase shifter film. Then, the attenuating type phaseshifter film is etched using the resist film as a mask.

The attenuating type auxiliary phase shifting pattern has a patternhaving a resolution smaller than the limit resolution of the exposureapparatus.

Preferably, forming the attenuating type phase shifter film includes thesteps of forming a semi-light shielding film having a lighttransmittance of 5-20% and forming a phase shifter film shifting a phaseof transmitted light by 180°.

More preferably, forming the semi-light shielding film includes the stepof forming a chromium film, and forming the phase shifter film includesthe step of forming a silicon oxide film.

According to the method of manufacturing an attenuating type phaseshifting mask, any images cannot be formed on the semiconductor wafer bythe light transmitted through the attenuating type auxiliary phaseshifting pattern because the resolution of the pattern is smaller thanthe limit of resolution of the exposure apparatus. Further, since beamsof light transmitted through the transmitting portion and beams of lighttransmitted through the phase shifter portion overlap with each otherand are in reverse phases, those beams cancel with each other due tointerference. Thus, the light intensity on the semiconductor wafer canbe reduced.

More preferably, forming the attenuating type phase shifter filmincludes the step of forming a film of a kind selected from a groupcomprising chromium oxide, chromium nitride oxide, molybdenum silicideoxide, and nitride oxide of molybdenum silicide.

Thus, the number of manufacturing steps of the attenuating type phaseshifting mask can be reduced because the attenuating type phase shifterfilm is formed of one kind of film, so that the manufacturing cost ofthe attenuating type phase shifting mask can be reduced.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) shows an attenuating type phase shifting mask according to thefirst embodiment viewed from the pattern formation surface, and FIG.1(B) is a cross sectional view along line X--X of FIG. 1(A).

FIG. 2 shows patterns of an attenuating type auxiliary phase shiftingpattern of the first embodiment.

FIG. 3(A) is an enlarged cross sectional view of the attenuating typeauxiliary phase shifting pattern, FIG. 3(B) is a cross sectional viewshowing an electric field of the exposure light directly under thephotomask substrate, FIG. 3(C) shows the electric field of the exposurelight on the semiconductor wafer, and FIG. 3(D) shows the intensity ofthe exposure light on the semiconductor wafer.

FIG. 4 is a graph showing the relationship between the ratio of theplanar area (S₀) of the transmitting portion to the planar area (S_(H))of the phase shifter (S_(H)) and the light intensity ratio on the wafer.

FIG. 5 is a graph showing the relationship between transmittance of thephase shifter and the value of a representing the range of the planararea of the phase shifter.

FIG. 6(A) shows the attenuating type phase shifting mask of the firstembodiment when viewed from the pattern formation surface, FIG. 6(B) isa cross sectional view showing positions of the attenuating type phaseshifting mask and a blind, FIG. 6(C) shows the electric field of theexposure light directly under the photomask substrate, and FIG. 6(D)shows the light intensity on the semiconductor wafer.

FIGS. 7-11 are sectional views showing the first through fifth steps ofmanufacturing the attenuating type auxiliary phase shifting patternaccording to the first embodiment of the invention.

FIG. 12 shows patterns of an attenuating type auxiliary phase shiftingpattern of the second embodiment.

FIG. 13(A) shows an attenuating type phase shifting mask of the thirdembodiment when viewed from the pattern formation surface, FIG. 13(B)shows patterns of an attenuating type auxiliary phase shifting patternof the third embodiment, and FIG. 13(C) shows patterns of an attenuatingtype auxiliary phase shifting pattern of the fourth embodiment.

FIG. 14 is a first view illustrating a problem occurred in respectiveembodiments.

FIG. 15 is a second view illustrating a problem occurred in respectiveembodiments.

FIG. 16(A) is a cross sectional view showing a structure of aconventional photomask, FIG. 16(B) shows the electric field of theexposure light on the mask, and FIG. 16(C) shows the intensity of theexposure light on the wafer.

FIG. 17(A) is a cross sectional view showing a structure of aconventional phase shifting mask, FIG. 17(B) shows the electric field ofthe exposure light on the mask, and FIG. 17(C) shows the intensity ofthe exposure light on the wafer.

FIG. 18(A) shows a structure of a conventional attenuating type phaseshifting mask, FIG. 18(B) shows the electric field of the exposure lighton the mask, and FIG. 18(C) shows the intensity of the exposure light onthe wafer.

FIG. 19 is a graph showing the relationship between transmittance andthe thickness of the resist film after development.

FIG. 20(A) is a cross sectional view showing positions of theconventional attenuating type phase shifting mask and the blind, FIG.20(B) shows the electric field of the exposure light directly under thephotomask substrate, FIG. 20(C) shows the intensity of the exposurelight on the semiconductor wafer, and FIG. 20(D) shows the state ofexposure on the semiconductor wafer.

FIG. 21 shows problems encountered when the conventional attenuatingtype phase shifting mask is used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of an attenuating type phase shifting maskaccording to the present invention will be described below.

FIG. 1(A) shows an attenuating type phase shifting mask 1 of the presentembodiment when viewed from pattern formation side. FIG. 1(B) is a crosssectional view along line X--X of FIG. 1(A).

In attenuating type phase shifting mask 1, an attenuating type phaseshifting pattern 2, which is a square region, is formed approximately atthe center on a photomask substrate 4. An attenuating type auxiliaryphase shifting pattern 3 is formed at the entire periphery of theattenuating type phase shifting pattern 2.

An attenuating type phase shifting pattern 2 is formed by a phaseshifter portion 24 and a transmitting portion 27. Phase shifter portion24 is formed by chromium film 2a having a transmittance of 5-20% andSiO₂ film 2b providing phase shifting of 180°.

Referring to FIG. 2, an attenuating type auxiliary phase shiftingpattern 3 will be described below. Attenuating type auxiliary phaseshifting pattern 3 is formed by processing a phase shifter portion 34formed by chromium film 3a having a transmittance of 5-20% and SiO₂ film3b providing phase shifting of 180° into a pattern of a size smallerthan the limit of resolution of the exposure apparatus. In FIG. 2, aplanar area of phase shifter portion 34 is represented by S_(H) and aplanar area of a square transmitting portion 37 is represented by S₀.

Referring to FIGS. 3(A) to 3(C), the light intensity on thesemiconductor wafer of the exposure light transmitting throughattenuating type auxiliary phase shifting pattern 3 will be described.

Referring to FIG. 3(A), phase shifter 34 and transmitting portion 37 areformed into predetermined shapes on photomask substrate 4 with theplanar area of the phase shifter 34 represented by (S_(H)) and theplanar area of transmitting portion 37 represented by (S₀).

Referring to FIG. 3(B), the electric field of the exposure lightdirectly under photomask substrate 4 has the transmittance and the phasethereof converted according to patterns formed. As to the electric fieldon the exposed material of the exposure light transmitted throughtransmitting portion 37 and phase shifter 34, when optical images areprojected through a demagnification lens, as shown in FIG. 3(C), theelectric field on the exposed material of the exposure light transmittedthrough transmitting portion 37 has an approximately constant value asshown by curve f, while the electric field on the exposed material ofthe exposure light transmitted through phase shifter 34 also has anapproximately constant value as shown by curve g similarly to curve f,because the patterns are below the limit of resolution and images areoverlapped with each other.

Thus, as shown in FIG. 3(D), the intensity of the exposure light on theexposed material is reduced, since electric fields shown by curve f andcurve g are canceled with each other.

If absolute values of electric fields shown by curve f and curve g areequal, the light intensity on the exposed material can be minimized.

The magnitude of electric fields shown by curve f and curve g can bedetermined respectively according to the relationship between planararea (S_(H)) of phase shifter 34, planar area (S₀) of transmittingportion 37, and transmittance (T) of phase shifter 34.

In FIG. 4, abscissa indicates the S₀ /S_(H) ratio, and ordinateindicates the ratio I/I₀ (%) of the light intensity on the wafer.

The solid line represents the case when the transmittance of the phaseshifter film is T=0.075 (7.5%), while the dotted line represents thecase when the transmittance of the phase shifter film is T=0.12 (12%).

As can be seen from FIG. 4, the minimum value is obtained with S₀ /S_(H)=0.25 when the phase shifting film having the transmittance T=7.5% isused, while the minimum value is obtained with S₀ /S_(H) =0.35 when thephase shifter film having the transmittance T=12.0% is used, and theI/I₀ ratio becomes not more than 0.1% in either case.

Accordingly, in order to attain the ratio I/I₀ of not more than 3%, therelationship between planar area (S₀) of phase shifter 34, planar area(S_(H)) of the transmitting portion 37, and transmittance (T) of thephase shifter 34 should satisfy the following relationship:

    S.sub.0 /S.sub.h ≈√T                        (2)

The acceptable range of the value √T will be described.

FIG. 5 is a graph in which abscissa indicates transmittance T% of thephase shifter 34, and ordinate indicates the value a when the range ofplanar area (S₀) of phase shifter 34 is set to (1-a) S₀ ˜(1+a) S₀.

Since there is a correlation between S₀ and S_(H), the value of S_(H)changes according to the value of S₀.

Consequently, the ratio S₀ /S_(H) can be represented by the followingequations: ##EQU1##

Therefore, it holds: ##EQU2##

Next, attenuating type auxiliary phase shifting pattern 3 will bedescribed in case, for example, where i-line (λ=365 nm) is used as thelight source and the demagnification exposure apparatus having 1/5 ofdemagnification ratio of the lens with NA=0.54 and k₁ =0.4 is used.

Referring to FIG. 2, square patterns are formed at a pitch of 2.0 μmwith the size of an aperture of transmitting portion 37 being 1.0 μm×1.0μm. The attenuating type auxiliary phase shifting pattern 3 is formed tohave the width of 1500 μm at the entire periphery of the attenuatingtype phase shifting pattern 2, as shown in FIG. 1.

Phase shifter 34 has a two-layered structure including a SiO₂ film 2bcontrolling the phase and a Cr film 2a controlling the transmittance, ofwhich phase shifting angle is set to 180° and the transmittance is setto 12%.

The limit of resolution of the above-mentioned demagnificationprojection exposure apparatus is set to 0.4 μm on the semiconductorwafer, while the limit is set to 2.0 μm on the photomask which is fivetimes that on the semiconductor wafer. Thus, attenuating type auxiliaryphase shifting pattern 3 is sufficiently smaller than the limit ofresolution.

Under the above-mentioned conditions, S₀ /S_(H) equals 0.33, and aequals 0.25 when transmittance is 12% according to FIG. 5, so that whenwe substitute these values, equation (4) would be as follows:

    0.23910≧S.sub.0 /S.sub.H ≧0.474

Since the value of √T equals 0.346, the relationship of equation (4) canbe satisfied and the relationship of equation (2) can be almostsatisfied.

Exposure utilizing attenuating type phase shifting mask 1 having theabove-described structure will be described below.

FIG. 6(a) shows an attenuating type phase shifting mask 1 wherein anattenuating type auxiliary phase shifting pattern 3 satisfying the aboveconditions is formed at the entire periphery of attenuating type phaseshifting pattern 2.

FIG. 6(b) is a sectional view showing the position of attenuating typephase shifting mask 1 relative to a blind 7 of the exposure apparatus.FIG. 6(c) shows an electric field of the light directly under aphotomask substrate. FIG. 6(d) is a graph showing the light intensity onthe exposed material.

Beams of light transmitted through attenuating type auxiliary phaseshifting pattern 3 do not form any images on the semiconductor wafersince the pattern is smaller than the limit of resolution, and at thesame time, beams of light transmitted through the transmitting portionand beams of light transmitted through the phase shifter are canceledwith each other resulting in the light intensity of not more than 3% ofthe light intensity before transmitting through the pattern, so thatregions other than the exposure region are not exposed.

Thus, exposure can be carried out satisfactorily even in the case ofsuccessive exposure as before.

Next, description will be made on a method of manufacturing theattenuating type phase shifting mask with referring to FIGS. 7-11.Referring to FIG. 7, chromium film 23a having a thickness of about 200 Åis formed on a quartz glass substrate 4. SiO₂ film 23a having athickness of about 4000 Å is formed on chromium film 23a. An electronbeam resist film (e.g. ZEP-810) 25 having a thickness of about 5000 Å idformed on SiO₂ film 23b.

Referring to FIG. 8, EB (Electron Beam) lithography is conducted onelectron beam resist film 25 using a variable shaped electron beamexposure apparatus (e.g. NEC JBX-7000MV, 6AIII). EB lithography isexecuted as below in order to obtain an accurate attenuating typeauxiliary phase shifting pattern.

More particularly, an exposure pattern of the attenuating type auxiliaryphase shifting pattern is set to have dimensions smaller than those ofthe finished pattern, or a dosage of electron beams directed to theattenuating type auxiliary phase shifting pattern region is made greaterthan that directed to the attenuating type phase shifting patternregion, or both steps are carried out. For instance, attenuating typephase shifting pattern region 200 is exposed at a dosage of electronbeams within the range of 8-10 μc/cm². In order to obtain apertures eachhaving dimensions of 1 μm×1 μm □ at intervals of 2 μm through exposureof attenuating type auxiliary phase shifting pattern region 300 by EBlithography, an exposure pattern should have dimensions of 0.8 μm×0.8 μm□. Electron beams are directed at a dosage of 10-12 μc/cm² in anoverdosed manner, whereby apertures having dimensions of 1 μm×1 μm □ canbe obtained. Electron beam resist film 25 is developed after EBlithography is completed, and then a predetermined pattern is obtained.

Referring to FIG. 9, SiO₂ film 23b is etched using electron beam resistfilm 25 having the predetermined pattern formed thereon as a mask.Etching is conducted with a magnetron RIE apparatus using CHF₃ +O₂ (CHF₃: O₂ =90:10) as an etching gas with an RF power of 200 W, a magneticfield of 100 G, and a gas pressure of 50 mtorr. Whereby a SiO₂ film 2bhaving a predetermined shape of pattern is formed at attenuating typephase shifting pattern region 200, and a SiO₂ film 3b having apredetermined shape is formed at attenuating type auxiliary phaseshifting pattern region 300.

Referring to FIG. 10, chromium film 23a is etched using again electronbeam resist film 25. As in the case of above-mentioned SiO₂ film,etching is conducted with the magnetron RIE apparatus using Cl₂ +O₂ (Cl₂: O₂ =80:20) with an RF power of 100 W, a magnetic field of 100 G andgas pressure of 50 mtorr. Thus, chromium film 2a of a predeterminedshape is formed at attenuating type phase shifting pattern region 200,and chromium film 3a of a predetermined shape is formed at attenuatingtype auxiliary phase shifting pattern region 300. Next, referring toFIG. 11, electron beam resist film 25 is removed, and thus attenuatingtype phase shifting mask 1 according to the present embodiment iscompleted.

In this embodiment, although a two-layer structure of SiO₂ film forcontrolling phases and chromium film for controlling a transmittance isemployed, a single layer film made of one kind of material selected froma group comprising chromium oxide, chromium nitride oxide, nitridecarbide oxide of chromium, molybdenum silicide oxide, and nitride oxideof molybdenum silicide may be used to control the phase andtransmittance to predetermined values. In this case, chromium oxide andthe like can be thinner to have a film thickness of about 1200-1600 Åcompared with the above mentioned two-layer structure, therebyfacilitating the formation of the phase shifting pattern.

The second embodiment according to the present invention will bedescribed.

In the first embodiment, square patterns each having the size of 1.0μm×1.0 μm have been described as patterns for the attenuating typeauxiliary phase shifting pattern. In such square patterns, the phaseshifter film is provided integrally so that it has a superior adhesionto the photomask substrate. However, manufacturing the photomask, theelectron beam lithography is time consuming since a large number oftransmitting portions should be formed. Especially, the time taken formanufacturing becomes longer in the lithography system utilizing avariable shaped beam, since it depends on the shape of patterns.

In order to shorten the time taken for lithography, attenuating typeauxiliary phase shifting pattern 3 is formed to have lines and spaceswith transmitting portions 37 and phase shifters 34 being linear andarranged alternately as shown in FIG. 12. In this case, the pattern 3 isformed at the entire periphery of attenuating type phase shiftingpattern 2 with the width of the transmitting portion being 0.5 μm, as inthe first embodiment. The cross sectional view along line Z--Z of FIG.12 is similar to the cross sectional view shown in FIG. 3(A).

The value of S₀ /S_(H) in the attenuating type phase shifting maskhaving the above-mentioned structure is 0.35 satisfying √T=√0.12≈0.33,so that equations (2) and (4) can be satisfied.

Therefore, the similar effect can be obtained as in the first embodimentby utilizing the attenuating type phase shifting mask according to thesecond embodiment.

An attenuating type phase shifting mask according to the thirdembodiment of the present invention will be described.

In the above-described first and second embodiments, the attenuatingtype auxiliary phase shifting pattern is provided at the entireperiphery of the attenuating type phase shifting pattern. However, ifthe transmittance of the absorption type shifter film forming the phaseshifter is small, for example, 5-8%, exposure of regions other than thedesired regions once by the light transmitted through the phase shiftermay be acceptable. Thus, when the transmittance of the phase shifter issmall, problem arises only at the region 32 shown in FIG. 21.

Accordingly, the attenuating type auxiliary phase shifting pattern areprovided only in the vicinity of four corners of the attenuating typephase shifting pattern in the present embodiment.

FIG. 13(A) shows an attenuating type phase shifting mask 1 according tothe third embodiment when viewed from the pattern formation surface,where an attenuating type auxiliary phase shifting pattern 3 is providedonly in the vicinity of four corners of attenuating type phase shiftingpattern 2.

In the present embodiment, the phase shifter of the attenuating typephase shifting pattern is formed of an absorption type shifter filmhaving the transmittance T of 7.5%, and the phase shifter is formed withthe length of one side being 2000 μm at each of the four corners of theshifting pattern. In each phase shifter, square patterns havingapertures of 0.9 μm×0.9 μm each has the transmitting portion 37 areformed at a pitch of 2.0 μm.

Under the above-mentioned conditions, S₀ /S_(H) equals 0.25, and aequals 0.3 if the transmittance is 7.5% according to FIG. 5, and when wesubstitute these values, equation (4) would be as follows:

    0.1771≧S.sub.0 /S.sub.H ≧0.38789

Since √T is 0.27, equation (4) can be satisfied and also equation (2)can be almost satisfied.

When exposure is carried out utilizing attenuating type phase shiftingmask 1 according to the third embodiment, even if the amount of exposurelight is four times the most suitable amount of exposure light,reduction of the thickness of the resist film corresponding to fourcorners of the attenuating type phase shifting pattern is not observed.

The fourth embodiment according to the present invention will bedescribed.

In an attenuating type phase shifting mask 1 according to the fourthembodiment, patterns formed on the attenuating type auxiliary phaseshifting pattern in the third embodiment are changed into lines andspaces, as shown in FIG. 13(C), with transmitting portions 37 and phaseshifters 34 being linear and arranged alternately. The width oftransmitting portion 37 is 0.5 μm and the pitch is 2.0 μm. Since S₀/S_(H) equals 0.25 satisfying √T=√0.075=0.27, equations (2) and (4) canbe satisfied.

In the fourth embodiment, similar effect can be obtained as in the thirdembodiment.

In the above-described embodiments, dimensions of the peripheral patternof the exposure region sometimes become smaller as shown in FIG. 14 ornarrower as shown in FIG. 15 during exposure of the attenuating typeauxiliary phase shifting pattern. This is because the amount of electronbeams directed to the peripheral pattern portion of the exposure regionis insufficient due to the proximity effect of exposure using electronbeams.

In order to avoid such a problem, a dimensional bias (+0.1-0.3) is addedto the data of the electron beam exposure at the time of exposing theperipheral pattern with electron beams.

Rectangular and linear shapes have been employed for the attenuatingtype phase shifting pattern, but circular or polygonal shapes may beutilized to achieve a similar function and effect as long as thoseshapes satisfy S₀ /A_(h) nearly equal √T.

Although the attenuating type auxiliary phase shifting pattern has beenprovided at the periphery of the chip in the above embodiment, theattenuating type auxiliary phase shifting pattern may be provided withinthe chip if the light shielding portion is necessary, by applying thesimilar method.

The exposure method utilizing the attenuating type phase shifting maskaccording to the above-described embodiments can be utilized effectivelyin the manufacturing process of DRAM, SRAM, flash memory, ASIC,microcomputer, and a semiconductor device such as GaAs of 4M, 16M, 64M,or 256M. Further, such a method is sufficiently applicable to asemiconductor device or a manufacturing process of a liquid crystaldisplay.

According to the attenuating type phase shifting mask of the presentinvention, beams of light transmitting through the attenuating typeauxiliary phase shifting pattern do not form any images on thesemiconductor wafer since the pattern is smaller than the limit ofresolution. Since beams of light transmitted through the transmittingportion and beams of light transmitted through the phase shifter overlapwith each other and the phase of the light transmitted through the phaseshifter is reversed, those beams of light are canceled with each other,whereby the light intensity on the semiconductor wafer can be reduced.Therefore, exposure of the portion other than the exposure region isprevented, the quality of exposure at the time of manufacturing thesemiconductor device is improved, and the yield of manufacturing thesemiconductor device can be improved.

Also, by setting values of planar area (S₀) of the transmitting portion,and planar area (S_(H)) of the phase shifter, and transmittance (T) ofthe phase shifter of the attenuating type auxiliary phase shiftingpattern to predetermined values it becomes possible to control the lightintensity on the semiconductor wafer by adjusting the intensity of thelight transmitting through the phase shifter and the intensity of thelight transmitting through the transmitting portion.

On the other hand, by setting S_(O) /S_(H), which is a ratio of planararea (S₀) of the transmitting portion to planar area (S_(H)) of thephase shifter of the attenuating type auxiliary phase shifting pattern,to be approximately equal to the value of √T of transmittance (T) of thephase shifter, the light intensity on the semiconductor wafer can becontrolled to be not more than 3% of the light intensity beforetransmitting through the pattern.

Further, the auxiliary phase shifting pattern is provided at the entireperiphery of the attenuating type phase shifting pattern. Thus, even ifa plurality of portions are exposed successively utilizing theattenuating type phase shifting mask, other regions are not exposed sothat satisfactory exposure can be carried out.

Further, the auxiliary phase shifting patterns are provided only in thevicinity of four corners of the attenuating type phase shifting patternhaving a rectangular shape.

Thus, when exposure is carried out successively in order on thesemiconductor wafer utilizing the attenuating type phase shiftingpattern, any region is not exposed more than once by the lighttransmitting through the periphery of the attenuating type phaseshifting pattern, so that satisfactory exposure can be carried out.

The shape of patterns having apertures in the transmitting portionformed on the attenuating type auxiliary phase shifting pattern isrectangular. Thus, adhesive property of the phase shifting pattern tothe photomask substrate is improved so that the attenuating type phaseshifting mask having a highly reliable structure can be provided.

The transmitting portions and the phase shifters formed on theattenuating type phase shifting pattern are linear and arrangedalternately. Thus, the attenuating type auxiliary phase shifting patterncan be formed easily so as to facilitate manufacturing of theattenuating type phase shifting mask.

According to a method of manufacturing an attenuating type phaseshifting mask, any images cannot be formed on a semiconductor wafer bythe light transmitted through an attenuating type auxiliary phaseshifting pattern because the resolution of the pattern is smaller thanthe limit of resolution of the exposure apparatus. Further, since beamsof light transmitted through a transmitting portion and beams of lighttransmitted through a phase shifter portion overlap with each other andare in reverse phases, those beams of light cancel with each other dueto interference, whereby the light intensity on the semiconductor wafercan be reduced. This prevents exposure of portions other than exposureregion during conducting exposure, improves exposure state duringmanufacturing of a semiconductor device, and further improves an yieldof manufacturing of the semiconductor device.

More preferably, forming the attenuating type phase shifter filmincludes the step of forming a film of a kind selected from a groupcomprising chromium oxide, chromium nitride oxide, nitride carbide oxideof chromium, molybdenum silicide oxide, and nitride oxide of molybdenumsilicide.

Thus, the number of manufacturing steps of the attenuating type phaseshifting mask can be reduced because the attenuating type phase shifterfilm is formed of one kind of film, so that the manufacturing cost ofthe attenuating type phase shifting mask can be reduced.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An attenuating type phase shifting mask,comprising:an attenuating type phase shifting pattern formed at apredetermined position on a photomask substrate; and an attenuating typeauxiliary phase shifting pattern including a transmitting portion and aphase shifter formed at a predetermined position at the periphery ofsaid attenuating type phase shifting pattern, said attenuating typeauxiliary phase shifting pattern having a resolution smaller than thelimit of resolution of the exposure apparatus.
 2. The attenuating typephase shifting mask according to claim 1, whereinin said attenuatingtype auxiliary phase shifting pattern, values of (a) planar area of saidtransmitting portion, (b) planar area of said phase shifter, and (c)transmittance of said phase shifter are set such that the lightintensity on the exposed material resulting from the light transmittingthrough said transmitting portion and the light intensity on the exposedmaterial from the light transmitting through said phase shifter beingcanceled with each other to an extent that intensity is reduced to notmore than 3% of the light intensity before transmitting through saidtransmitting portion and said phase shifter.
 3. The attenuating typephase shifting mask according to claim 2, whereinratio of planar area ofsaid transmitting portion to planar area of said phase shifterrepresented as S_(O) /S_(H) of said attenuating type auxiliary phaseshifting pattern is approximately equal to a value √T of transmittanceof said phase shifter.
 4. The attenuating type phase shifting maskaccording to claim 1, whereinsaid attenuating type auxiliary phaseshifting pattern is provided at the entire periphery of said attenuatingtype phase shifting pattern.
 5. The attenuating type phase shifting maskaccording to claim 1, whereinthe shape of said attenuating type phaseshifting pattern is rectangular, and said attenuating type auxiliaryphase shifting pattern is provided in the vicinity of four corners ofsaid attenuating type phase shifting pattern.
 6. The attenuating typephase shifting mask according to claim 1, whereinthe planar shape ofsaid transmitting portion of said attenuating type auxiliary phaseshifting pattern is rectangular.
 7. The attenuating type phase shiftingmask according to claim 1, whereinsaid transmitting portion and saidphase shifter of said attenuating type auxiliary phase shifting patternare linear and arranged alternately.
 8. A method of manufacturing anattenuating type phase shifting mask, comprising the steps of:forming anattenuating type phase shifter film having a light transmittance of5-20% on a transparent substrate for shifting a phase of a transmittedlight by 180°; forming on said attenuating type phase shifter film aresist film including an attenuating type phase shifting pattern regionand an attenuating type auxiliary phase shifting pattern region formedat a predetermined position at the periphery of the attenuating typephase shifting pattern region; and patterning said attenuating typephase shifter film by etching using said resist film as a mask; whereinsaid attenuating type auxiliary phase shifting pattern region includes apattern having a resolution smaller than a limit of resolution of anexposure apparatus.
 9. A method of manufacturing an attenuating typephase shifting mask according to claim 8, whereinsaid step of formingthe attenuating type phase shifter film includes the steps of: forming asemi-light shielding film having a light transmittance of 5-20%; andforming a phase shifter film shifting a phase of transmitted light by180°.
 10. A method of manufacturing an attenuating type phase shiftingmask according to claim 9, whereinsaid step of forming the semi-lightshielding film includes the step of forming a chromium film, and saidstep of forming the phase shifter film includes the step of forming asilicon oxide film.
 11. A method of manufacturing an attenuating typephase shifting mask according to claim 8, whereinsaid step of formingthe attenuating type phase shifter film includes the step of forming afilm of a kind selected from a group comprising chromium oxide, chromiumnitride oxide, nitride carbide oxide of chromium, molybdenum silicideoxide, and nitride oxide of molybdenum silicide.